As fibrous crops mature the yield of energy per unit of land area increases, but the availability of energy, i.e. cellulose and hemicellulose decreases because of the process of plant lignification in which the cellulose chains in the plant cell walls become lignified through complex cross linkages. This results in a considerable reduction in the digestibility of the dry matter components to the animal. Thus, whilst from an economic viewpoint, the crop should be harvested mature, at peak dry matter yield, the limitation on digestibility of energy forces harvest to take place at a more immature stage with a lower dry matter yield and potential problems with conservation due to lower dry matter content. By breaking such cross links between cellulose units, enzymes can allow crops to be harvested at later maturities, thus increasing energy yield without the usual reduction in digestibility. Also, the same process can make sugars available to silage microbes, thus improving the conservation of such crops under adverse weather conditions where dry matter content is low.
Conservation
The preservation of fibrous crops (grass, legumes, whole crop cereals--maize, sorghum, etc.) for future use as animal feed basically relies on either the removal of water by drying (hay etc.) or the exclusion of air and acidification of the mass to a point where the activities of epiphytic spoilage micro-organisms (yeasts, moulds and bacteria) are controlled and the enzyme activities of the plant material are restricted. In practice, a pH of less than about 4.2 is needed, this can be acheived either through the addition of acid to the crop or by means of acids produced through fermentation by epiphytic microorganisms.
Current environment concerns make the addition of acid less acceptable. The environmental concerns are amplified by the generally used acids such as formic acid. These acids increase the production of acidic effluent from wet crops.
The alternative is to rely on natural fermentation. However, natural fermentation produces variable results which are sometimes insufficient. For example, undesirable epiphytes may dominate the desirable lactic acid bacteria in crops that contain small amounts of sugar substrates. The desirable lactic acid bacteria, i.e. homolactic bacteria, produce mainly lactic acid. This lactic acid production lowers the pH without significant damage to the feed value, especially protein quality. In contrast, when undesirable epiphytes dominate, the pH is only slowly reduced and it may not reach a sufficiently low value. The resulting effect on feed quality is then detrimental to animal performance.
The situation can be improved by adding the desirable lactic acid bacteria to the crop at levels sufficient to dominate the epiphytes. But this technique does not help where the sugar level in the crop is low. The water soluble carbohydrates required by the lactic acid bacteria may be added to the crop. For example, one could add molasses, starch or sugars such as glucose, lactose and sucrose. However, this approach creates other problems. Generally, a large amount of sugar must be added, i.e. 10-20 kilograms sugar/tonne and between 35 to 45 kilograms molasses/tonne crop. It is difficult to apply such high quantities of these viscous materials to the crop. It is also difficult to evenly add dry materials to the crop. Some additives, such as glucose, lactose and sucrose, are too expensive to use. Moreover, lactic acid bacteria do not use starch effectively unless amylase is present to convert the starch to sugar.
Alternatively, enzymes can be used to break the complex structural carbohydrates in the crops into simple sugars. Lactic acid bacteria can use the sugars released this way and dominate fermentation. U.S. Pat. No. 4,751,089, to Heikonen et al., recites a method for ensiling fodder and grain by adding glucose oxidase. The glucose oxidase produces gluconic acid from glucose in the soluble carbohydrates. The gluconic acid accumulation decrease the pH. According to said U.S. patent other enzymes, such as cellulase, hemicellulase and B-glucosidase can be added to increase the glucose production.
The use of cellulolytic enzymes to preserve and enhance the nutritive value of forage for silage and to improve the palatability, digestibility and rate of digestion of treated forage by ruminants has also been described in U.S. Ser. No. 510,506, filed on Apr. 18, 1990, now abandoned. The enzyme composition disclosed in that application preferably contains at least one enzyme from the group consisting of pectinase, cellulase, xylanase, amylase, arabinosidase, cutinase, lipase and esterase and may be used in combination with homolactic bacteria.
However, cellulolytic enzymes can produce undesirable side effects when added to fibrous crops having little (i.e. less than 25%) dry matter such as immature crops. For example, cellulolytic enzymes can increase the amount and pattern of effluent flow. The effluent flow comprises soluble cellular materials. These materials give the effluent a high BOD and can cause environmental problems. Also, the effluent loss reduces the feed value of the crop. These enzymes can also increase the lactic acid values and change fiber structure to a degree that reduces animal performance. Moreover, as a result of the increased sugar levels produced by the enzymes, yeasts and moulds may grow better. Increased yeast and mould growth may lower the aerobic stability and produce harmful mycotoxins.
Feed Efficiency
In ruminants the efficiency with which fiber is used by the host animal depends on the effective actions of a mixed rumen microbial population. The composition of this microbial population depends upon the feed. The end result of microbial activity on energy sources is the production of volatile fatty acids which act as precursors within the tissue of the host for the supply of energy for metabolic processes and for the synthesis of animal products e.g. milk, meat and wool. The efficiency with which these products are produced depends on the relative proportions of the volatile fatty acids, especially acetic, propionic and butyric. Feeds with a high starch and/or sugar content promote the synthesis of butyric and propionic acid whereas fiber promotes acetic acid. The desirable type of rumen fermentation depends upon the animal product required. Thus, the ability to modify the substrate is of prime economic importance, especially the ability to modify the reactivity of fiber in this respect, since this is the lowest cost form of energy.
For monogastric animals, fiber is not a ready source of energy but it is present in most sources of starch, e.g. grains. Fiber is also of importance in maintaining the normal gut function. This is associated with the reactivity of the fiber, e.g. cation exchange capacity. The ability to release energy from the fiber fraction of the diet and to improve its reactivity is thus of great importance in monogastric nutrition and health.
Enzyme products for the preservation of low dry matter forage and the enhancement of feed utilization present two major problems. Firstly, in an efficient ensilage process, the enzyme should produce the desired pH, lactic acid and carbohydrate concentration while minimizing the effluent production.
The treatment of forage with a complete mixture of cellulolytic enzymes decreases the fiber content of the silage by solubilizing polymeric carbohydrates. Over effective digestion results in total cell wall collapse and consequently in production of effluent with high sugar content. Fermentation during ensilage is stimulated, lactic acid accumulates and pH drops. Under these conditions bacteria are inhibited but the enzymes keep producing monomeric carbohydrates, part of which may be lost with the effluent. The silage containing high concentrations of lactic acid and easily fermentable sugars may be harmful to the ruminant, causing lactic acidosis and digestive disorders.
Secondly, in order to ensure efficient rumen function and feed utilization by ruminants, the amount of sugar available to the rumen microbes and the reactivity of the fiber should be optimized.
The object of this invention is to provide an enzyme preparation for different crops, maturities and dry matter content which does not have the disadvantages of the known preparations. More particularly, the object of this invention is to provide enzyme combinations which; give beneficial changes in the structure of plant cells walls; provide only the needed amount of sugars for an effective silage; not increase the production of effluent; not encourage yeast or mould growth; but which are able to change the structure of the plant polymers so that they are more susceptible to further enzymatic hydrolysis in the rumen and have improved digestion in the monogastic digestive tract.
A further object of this invention is to provide methods for the preparation of said enzyme products.